1. In the Australian Management Plan for Pandemic Influenza, a pandemic was defined as the arrival into Australia of a novel sub-type of influenza to which the population had little or no immunity. The WHO pandemic definition was infection with a novel virus resulting in substantial morbidity and mortality. The H1N1 2009 virus was novel but was not a novel subtype. With notable and important exceptions, the pandemic has not been associated with substantial morbidity and mortality. This pandemic has not fitted the notion of a pandemic in our pandemic plans.
2. Using the rate of influenza-like illness in sentinel general practice patients as a guide to assessing the 2009 pandemic, we can conclude that 2009 resembled the influenza seasons of 2003 and 2007, seasons characterised as higher-than-normal seasonal activity. In Victoria, the proportion of influenza-like illness confirmed as influenza in 2009 was about 39%, not different to the proportion between 2003-07. Almost all influenza detections in 2009 were swine flu.
3. However increased publicity associated with the pandemic has meant that the recorded rate of influenza-like illness may have been increased in 2009 by presentations to general practitioners that might not have occurred in another year. When we attempt to adjust for increased presentations and increased testing, the 2009 pandemic appears to more closely resemble the normal influenza seasons of 2004 and 2006.
4. Children and young adults have been the groups most affected. The median age of infection has been reported as 21 years. Although accurate clinical attack rates (the proportion of the population who get symptomatic influenza in a season) have yet to be determined, an estimate from New Zealand suggested a clinical attack rate of 7.5% and the best fit to a model for hospitalisations in Victoria was consistent with a clinical attack rate of 5%. Attack rates for seasonal influenza are thought to be in the range 1-5%.
5. A pandemic paradox, not yet explained, is that while disease in the community was mild and the risk of hospitalisation was relatively low, a high proportion of hospitalised patients required intensive care, and some required this for a very long time.
6. Pregnant women were at increased risk of hospitalisation, intensive care admission and death compared to other members of the population and to non-pregnant women of the same age. This increased risk appeared to be over and above the recognised risk of seasonal influenza infection in pregnancy.
7. Indigenous Australians were also at increased risk of hospitalisation, intensive care admission and death compared to non-indigenous Australians. This is also true of the risk associated with seasonal influenza infection.
8. 185 deaths from pandemic influenza have been reported in Australia to date. This is very much less than the average of 3000 deaths modelled to occur from seasonal influenza each year. However deaths from pandemic influenza occurred at a median age of 53 years compared to the median age of death from seasonal influenza of 83 years in the years 2001-06.
9. Interventions such as school closures appear to have had little impact on containing the pandemic, probably due to the continued mixing of these age groups outside the school environment.
10. Volunteer studies suggest the majority of infections with influenza H1N1 viruses are likely to be without a fever. Thermal imaging at the borders was therefore always going to be ineffective at detecting all infected passengers. The World Health Organisation does not recommend border control as part of pandemic management and modelling suggests that border control needs to be draconian to have any chance of success.
11. Australia’s pandemic plan was predicated on a dramatic pandemic like 1918-19. Responses that may have been appropriate in this setting may not have been optimal for the 2009 pandemic.
12. Our understanding of seasonal influenza is limited by the lack of routine laboratory testing for influenza. This also impacts on our understanding of the effectiveness and cost-effectiveness of interventions intended to protect us from influenza infection. The pandemic has brought this lack of understanding into sharp relief.
Dr Heath Kelly is an Associate Professor at the University of Melbourne and the author of an article on pandemic influenza appearing in today’s edition of The Medical Journal of Australia